1. Field of the Invention
This invention relates to the general field of powder metallurgy and it is particularly concerned with a preliminary treatment of powders to be employed for compacting operations in order to improve on the quality of the articles so obtained.
2. Description of the Related Art
As is well known, the usual powder metallurgy practice employs raw materials in the form of powders suitably prepared, compacting such material by means of one of the various processes normally employed and then treating the product so obtained adopting suitable conditions of temperature, pressure and composition of the environmental atmosphere in order to obtain a product of desired physicochemical and technological characteristics.
It is well known to those who are skilled in the art that the final characteristics obtained remarkably depend on the density increase obtained by means of the forming operation. Because of that reason, many processes have been introduced into the industrial practice both for improving on the capability of raw materials of undergoing increase in density, and for obtaining improvements through the addition of some agents to the mixtures employed.
It is common practice to add to the mixtures of powders to be compacted some compounds that substantially are not necessary for obtaining the final product, but capable of giving some lubricating effect so as to reduce the forces necessary to give the desired density increase. Lubricants commonly employed are of various types: among the most widespread lubricants, lithium stearates, zinc stearates as well the stearates of other metals, paraffins, waxes, natural or synthetic fat derivatives are employed. Such lubricants are added to raw materials in amounts from 0.2 up to 10% by weight, according to various compacting or densifying techniques and according to the various final properties desired for the products obtained.
Powders so prepared are then subjected at room temperature to the selected forming process. Among the most widespread forming systems, the technique of compacting in press molds, extrusion, the employment of injection molds, and rolling can be mentioned. In all such cases, a certain pressure is exerted on the mixed powder, which as a consequence becomes denser but opposes to the pressure "P" a reaction force that increases almost exponentially with increasing the density and can be determined by the ratio "R" between the apparent density of the compacted powder and the theoretical density of the material employed.
The strong pressure exerted gives rise to remarkable frictional forces among the particles of the powder, as well as between the powder and the walls of the molds and of the forming equipment. Thus, some practical difficulties arise in compacting powder beyond a given limit, together with the difficulty of extracting the compacted powder out of molds because of the strong frictional forces with the walls, tendency to wear or to breaking of equipment, need for employing high quality powders which are much more expensive practical impossibility of producing articles of complicated shapes, and need for machines of higher power.
A process according to the usual technique for reducing such difficulties consists in employing powders heated up to 350.degree.-650.degree. C. in a non-oxidizing controlled atmosphere and poured into a mold containing a lubricant and previously heated up to 150.degree.-450.degree. C. for compacting said powders. However, such process is limited by the requirement of heating the powder up to a relatively high temperature and of employing a non-oxidizing atmosphere, as well as by the requirement of treating unlubricated powders and by the requirement of preliminarily heating the mold and of lubricating the same, in addition to the need for controlling the atmosphere around the mold.